Apparatus and method of heating powdered metals



April 26, 1966 L. BONIS ETAL APPARATUS AND METHOD OF HEATING POWDEREDMETALS TIMER RF GEN.

. INPUT REGULATOR INVENTORS d. BONIS LASZLO OLE- ANDREAS SANDVEN BERNARDMANNING smy ATTORNEYS United States Patent O 3,248,215 APPARATUS ANDIVLETHOD F HEATING POWDERED METALS Laszlo J. Bonis, Brookline,Ole-Andreas Sandven, Boston,

and Bernard Manning, Waltham, Mass., asslgnors to Ilikon Corporation,Natick, Mass., a corporation of Massachusetts Filed Sept. 26, 1963, Ser,No. 311,831 12 Claims. (Cl. 75226) Our invention concerns an improvedapparatus for and a method of heating powdered metals. In particular,our invention concerns a means of sintering and impregnating metals withlubricants. More particularly our invention relates to an apparatus forand method of forming articles from powdered metals containing heatsensitive dry lubricants.

The techniques of powder metallurgy have permitted powdered metals to becold or hot pressed into a variety of articles such as gears, brushes,sleeves, valves, bearings, shafts, bushings, etc. Sintering of the metalparticles either during or after compaction in a mold is accomplished atelevated temperatures below the fusion point of the powder forsufficient time to promote incipient fusion and the destruction of thegrain boundaries or until diffusion occurs between the metal particles.Sintering and elevated temperatures can be accomplished by employing themass of metal powders in an inductive relationship with a coil connectedto a high frequency generator. The currents set up within the mass ofmetal powder bring about a rapid rise in temperature of the metal massand the sintering of the mass.

Although the use of a conductive high frequency helix about theelectrically conductive mass of powdered metal is satisfactory for somepurposes it has disadvantages which inhibit wide commercial acceptanceof this method.

In many cases a dry lubricant must be admixed .with or impregnated intothe powder metal mass to provide a self lubricating article, containingfinely divided solid phase particles of dry lubricants uniformlydispersed through the solid sintered matrix. Many of the commerciallyacceptable dry lubricants which are desirable to blend with the metalpowder prior to sintering are chemically degraded at a progressivelyrapid rate as the temperature of the sintering increases. For example,molybdenum disulfide completely oxidizes above about 750 F., whiletungsten disulfide is completely oxidized at above about 850 F. Presenthigh frequency techniques due to the rapid temperature rise and uneventemperatures within the metal mass often tend to degrade and reduce theefiectiveness of heat sensitive dry lubricants.

Additionally the longer heating time and elevated temperatures obtainedwith high frequency coils also encourages reactions between the ambientgases such as nitrogen and oxygen with the metal mass. The formation ofmetal matrix oxides and materials is generally undesirable in powdermetallurgy, since they give lower density articles with inferiormechanical properties.

The degradation of heat and temperature sensitive lubn'cants and thereaction of the metal mass with atmospheric gases is further promoted byuneven heat distribution within the metal mass which is oftenaccompanied by a waste in electrical energy. For example, in a hotpressing operation there are no provisions made for maintaining uniformhigh frequency energy as the volume of the electrically conductiveparticles changes with comice paction in the mold. Since reductions involume of the packed particles in a mold may change on compaction from50 to volume percent or more depending upon the particles and theirgrain size, significant unevenness in heat distribution often occurs anda lack of uniform sintering temperatures through the compacted massleads to unsatisfactory molded articles with poor properties.

It is therefore an object of our invention to provide a method ofheating powdered metal compositions which minimizes chemical degradationof the composition.

Another object of our invention is to provide an apparatus for and amethod of maintaining efficient and uniform heat distribution in a metalmass during a hot pressing operation.

A further object of our invention is to provide means for formingpowdered metal articles containing heat sen sitive dry lubricants.

Other objects and advantages of our invention will be apparent 'to thoseskilled in the art from the following detailed description of ourinvention taken in conjunction with the accompanying drawing wherein:

FIGURE 1 is -a partially schematic and sectional view of an apparatusfor heating by a high frequency coil and compaction in a die of a massofmetal powder showing the position of the coil and mass prior tocompaction;

FIGURE 2 is a sectional view of a portion of the apparatus of FIGURE 1showing the position of the mass and coil after compaction.

In one embodiment of our invention we have discovered that effectiveheating and sintering of electrically conductive dielectric lossyparticles such as a mass of metal powder or doped plastics such asplastics containing carbon particles, etc. may be accomplished byemploying pulsating high frequency induction heating. We

have found that the employment of continuous high he quency energy to amass of metal particles for a period of time sufficient to reach propersintering temperatures (e.g. 500 to 2500 F.) permits the electricallyconductive metal particles to rapidly increase in temperature and totransfer all or a substantial portion of this heat to the heat sensitivematerials blended with the metal powders.

We have discovered that the use of a high frequency energy for shortperiods of time in .a cyclic manner such as from /2 second to 20 secondsor more permits elevated temperatures to be obtained in the metalpowders without significant heat transfer to the nonelectricallyconductive particles of the blend such as the dry lubricants. Selflubricating particles such as calcium fluoride, molybdenum -disulfide,tungsten disulfid-e, graphite, diamond dust, boron nitride, and othermetal sulfides and metal compounds are normally not electricallyconductive. The powdered metals of the matrix are electricallyconductive and should have sufiicient resistivity so that it is adielectric lossy material. The -heat transfer of the heat created by thehigh frequency energy in the metal particles to the dry lubricants andto the ambient atmosphere is significantly reduced, when the highfrequency is employed in a pulsating manner. This reduction in heattransfer permits d'ry lubricants and other materials normally degradedsuch as oxidized at conventional sintering temperatures to beincorporated into sintered articles Without excessive decomposition.Further, our technique reduces reactions between the atmosphere and themetal matrix.

The time of the pulses depends upon the nature of the materialsemployed, but we have found that relatively short periods of from 1 to10 seconds are often quite.

successful. Pulsing high frequency is usually employed in a hot pressingoperation, i.e., where compaction and sintering is accomplished in oneoperation. The mass of powdered metal is usually placed in anonconductive mold and the mold surrounded by a water cooled conductivehelix of copper. High frequency energy of from 1000 to to megacycles maybe used, but normally 10 to 1000 kilocycles are employed. The pulses maybe of uniform time or vary in time depending upon the heat transfercharacteristics of the mass being sintered and the sensitivity of thematerial being impregnated into the mass. For example, softer metalslike lead may require only 200 C. for sintering, while nickel requires1200 F. and tungsten 2000 C. The metal powder mixtures with or withoutadditional binder materials or additives may be compacted in a confiningmold under pressures of about 2000 to 50,000 pounds per square inch orhigher to form a metal powder article of predetermined structure. Incold pressing operations, that is without sintering, pressures of 35,000p.s.i. are often used, while in hot pressing operations pressures of 3to 15,000 are common. The compacted sintered article may be optionallyheat aged and slowly or rapidly quenched in a fluid such as air, oil,water, etc., to enhance the hardening characteristics of the article.

The mixtures described can be formed into a variety of improvedantifriction articles such as porous powdered metal articles, havingload bearing moving or non-moving frictional surfaces where one or moreof the surfaces or articles contain reservoirs or pockets of the drylubricants or other material. The articles formed from the dry powderedmixtures can be of any predetermined shape, size or characteristics andinclude brushes for electrical generators, contacting switches, gears,ashpots, valve stems, tubular elongated bearing sleeves, journal andthrust bearings, solid and tubular shafts, races, retainers,

bushings, etc., or other articles subject to frictional forces andhaving wearable destructible friction surfaces. In the articles soformed the dry lubricant may be evenly dispersed throughout the articlesuch as uniformly throughout the metal matrix. Where desired thelubricant may be incorporated only into particular areas of the articlesuch as the surface areas or other areas of the article subject toparticular severe frictional forces.

In one embodiment of the invention the matrix material comprises anyelectrical conductive material containing minor amounts ofnonelectrically conductive heat sensitive materials. For example,powdered metals, metal alloys and metal-containing compounds capable of.being compacted and sintered to form a solid metal-containing matrixsuch as by conventional powder metallurgical methods may be used. Themetals employed include those relatively soft metals, the hardest steeltool metals and metal alloys. The powdered metals employed can thusinclude those metals such as tin, lead, zinc, copper, nickel, iron,cobalt, aluminum, etc., and metal alloys containing trace or minorquantities of property modifying metal constituents such as copper, tin,zinc, lead, iron, chromium, cadmium, magnesium, palladium, platinum,rhodium, vanadium, nickel, cobalt, tungsten, molybdenum, aluminum,manganese, etc., and combinations thereof either with or without otheradditives and dry lubricants such as metal sulfides, boron nitride,graphite and the like.

The dry lubricants can be thoroughly admixed with the metal or metalalloy powder by hand or more efficiently by high-speed powder blendingmachines. Good results are commonly obtained by having the metal powderparticles of larger average diameter than the dry lubricant powder withwhich it is admixed. The quantity of the heat sensitive dry lubricant tobe admixed with the metal powder depends on the amount and degree oflubrication desired and the particular conditions under which thearticle is to be employed. Commonly the amount of heat sensitivenonelectrically conductive materials employed will vary from about 1weight percent to about 35 weight percent such as between 3 to 25 weightpercent of the matrix. Generally in selfduorieating metal alloycompositions the tensile strength elon gation, and compremive strengthincrease with decreasing amounts of dry lubricant, while as the amountincreases the coefiicient of thermal expansion and electricalresistivity increases and the hardness decreases.

In another embodiment of our invention we have found that uneven heatdistribution during a hot pressing operation can be substantiallyreduced or eliminated by varying the length of the high frequency coilabout the metal mass with the compaction of the metal. This techniquealso permits heat sensitive materials such as lubricants, antioxidants,resins, etc., to be blended with and impregnated into the powdered metalduring hot pressing and reduces undesired reactions between the sinteredmetal matrix and the pressing atmosphere. Our apparatus al-' lows eithercontinuous or pulsating high frequency energy to be employed. Pulsatingenergy and varying the coil length with compaction is particularlyuseful where ma terials are used that are highly sensitive to heat andthus our apparatus permits a wide rangeof new materials to be consideredfor use in powdered metallurgy.

FIGURE 1 shows a high frequency heating apparatus of our invention whichincludes: a high frequency RF generator 10 such as a 20 kilowattgenerator conncted to power input source (not shown), a timer 12 andaninput power regulator 14. The timer 12 permits RF energy to be generatedin a timed cyclic manner as heretofore described. The input powerregulator 14 or other vari= able means is used to control the generator10. In prac= tice the input power control 14 can be a powerstat betweenthe input rectifier tube and the primary of the generator transformer tocontrol the voltage to the primary and thus act as a variabletransformer. Our apparatus includes an elongated cylindrical moldcomposed of graphite, ceramic, quartz, or other electrically non-conductive material which forms an internal chamber in which electricallyconductive particles of lossy dielec" tric properties such as powdermetal 18 are loosely packed. The mold has at one end a top insert plug20 which rests against a non-movable anvil 22 and has at the other end aslidable reciprocating piston 24 mounted on a vertically movable anvil26. A hollow helix 28 of copper tubing or other RF conductive materialof a flexible nature is mounted extendably about the mold 16, with thelongitudinal axis of the coil the same as the axis of the mold 16, andthe length of the coil being the length of the loosely packed powderedmetal 18 in the mold 16. The coil or helix 28 is supported on eitherside, both top and bottom, by threaded elongated vertical support rods30, 32, 24, and 36, which at one end are fastened to either side of thecoil 28 through electrical insulator bushings 38. The other ends of eachpair of support rods are threadably attached to the movable 26 andnon-movable 20 anvils respectively, to permit rapid adjustment of thecoil length with different mold lengths. The coil is connected throughelectrical leads 40 and 42 to the output of the RF generator, while inoperation water or other fluid cooling medium is circulated through thehelix to prevent overheating. Secured to the base of the movable anvil26 through an eyelet screw is one end of a spring return pulley 44 theother end of which is fastened to the control knob of the power control14. This system permits the variation of the power of the RF generator10 with the movement of the piston 24 and anvil 26.

In a hot pressing operation powdered material such as metal powder 18with or without a heat sensitive material is loosely packed in the mold16, the top plug 20 inserted, and the top anvil 22 lowered into positionand the supportrods 34 and 36 attached thereto and the coil 28 adjustedto the length of the powder 18 in the mold 16. The,

powdered material is then compacted and sintered in one operation in themold, although it is also within the scope of our invention that thepowder may be compacted to a predetermined degree and then sintered orcompacted and sintered. Hot pressing is accomplished by the movableanvil 26 and the piston 24 moving slowly upwardly, while the sinteringis accomplished by the continually or pulsating high frequency energyfurnished to the coil 28 from the generator 10. In our apparatus as thepiston moves upwardly to compact the powder giving a progressivelyshorter length and small volume of powder in the mold, the longitudinallength of the surrounding coil 28 is correspondingly decreased orcompressed to maintain the same inductive relationship between the coil28 and the powder core 18, yielding better and more even heatdistribution and control. In our apparatus upper and lower electricallimit switches or other means may be used to automatically initiate andstop the flow of electrical energy to the coils depending upon the coilposition. Our invention allows the flexible heating coil such as a highfrequency coil described to vary in longitudinal length with the amountof compaction within the mold. For example, as shown more fully inFIGURE 2, the 50% compaction of the powdered metal 18 in the mold 16resulted in a corresponding de crease in the length of the coil 28. Thusin FIGURE 1 where the powder is loosely packed the coil 28 extendedthroughout its length, while in FIGURE 2 the powder is in a compacted ordense condition the coil 28 has been compressed to a shorter length tocorrespond to the length of the dense mass.

Additionally, even heat distribuition and superior heat control duringhot pressing can be accomplished by the variation of the power of thegenerator with the compaction of the material. Thus in operation as theanvil 26 moves upwardly to compact the powder 18 the pulley decreasesthe power of the generator 10 by moving the dial knob of a powerstat orcontrol knob on the energy control 14 to maintain even heat distributionthroughout the compacting mass. When the anvil moves downwardly thereturn spring of the pulley returns the energy to the same level. Asshown in the drawing the pulley decreases the energy output of thegenerator 10 as the anvil 26 compacts the powder 18 thus reducing the RFenergy to the coil28.

Further in our apparatus the timer of either a mechanical cam operatedor electrical type can be employed to give timed pulses of RF energy tothe coil 28 rather than continuous energy. The timer can be preset for/2 second to min. pulses depending on the nature of the material to behot pressed. The timer or the pulsation of high frequency energy can beused with the means for varying the output energy with compaction i.e.the pulley system, or may be used alone. The timer can also be operatedin response to limit switches on the anvils so that pulsation will startand stop depending on the position of the coil 28 about the mold 16.

Although our invention has been described with regard to metal powdersand high frequency coils it is of course recognized that our apparatuswill have utility with other materials such as powdered resins, glassand the like where electrical or other heating coils are employed toprovide elevated temperatures and the material change in volume duringthe heating operation.

What we claim is:

1. A hot pressing apparatus which includes: a source of energy; a moldcharacterized by an internal mold cavity capable of containing a looselypacked powdered material to be compacted; a heating coil incommunication with said source and about and in a heat exchangerelationship with the powdered material in the mold cavity; means tocompact the powdered material in the mold; and means to vary the lengthof the heating coil in heat exchange relationship with compaction of thematerial whereby uneven heat distribution during compaction is reduced.

2.- An apparatus for fabricating sintered articles which apparatuscomprises in combination:

a source of high frequency electrical energy;

a mold characterized by aninternal cavity adapted to contain particlesto be compacted and sintered; means to compact the particles in the moldcavity into the desired form;

a coil in electrical communication with said source of high frequencyelectrical energy, said coil encircling the mold cavity and in aninductive relationship with the particles in the mold cavity to permitsintering of the particles; and

means to change the longitudinal length of the coil during thecompacting of the particles.

3. The apparatus of claim 2 which includes timing means in electricalcommunication with the source of high frequency electrical energy toprovide for predetermined timed pulses of electrical energy to the coil.

4. The apparatus of claim 2 which includes means to vary the power ofthe high-frequency electrical energy supplied to the coil with thecompaction of the particles in the mold cavity.

5. An apparatus for fabricating sintered metal articles which apparatuscomprises in combination:

a source of high frequency electrical energy;

a mold characterized by an internal cavity adapted to contain powderedparticles to be compacted and sintered;

means to compact the particles in the mold cavity which includes aslideable reciprocating piston;

a deformable coil in electrical communcation with the source of highfrequency electrical energy, said coil spaced apart from and encirclingsaid mold cavity and in an inductive relationship with the particles inthe mold cavity;

means to support the coil about the mold cavity said means secured tosaid piston for movement therewith; and

means to impart reciprocating movement to said piston whereby thelongitudinal length of said coil about the mold cavity is proportionallyreduced as the particles within the mold cavity are compacted andsintered.

6. The apparatus of claim 2 which includes control means to control thepower of the source of high-frequency electrical energy and means toconnect the control means to the movement of the piston whereby theinput power is adjusted with the compaction of the particles in themold.

'7. In a method of forming a compacted sintered article which includesthe steps of compacting in a mold cavity a mass comprising electricallyconductive particles and sintering said particles during compacting byhighfrequency electrical energy from a coil placed in an inductiverelationship with the mass of particles the improvement which comprises:maintaining a substantially constant inductive relationship between thecoil and the mass of particles during the compaction and sintering byvarying the form of the coil thereby promoting more even heatdistribution within the compacted sintered article.

8. The method of claim 7 which includes changing the power of thehigh-frequency electrical energy progressively as the particles arecompacted.

9. The method of claim 7 which includes pulsating the high frequencyenergy during compacting in timed periods of from /2 second to 5minutes.

10. The method of claim 7 wherein the mass comprises powdered metalparticles and from about 1 to 30 weight percent of electricallynon-conductive dry lubricant particles.

11. The method of claim 10 wherein the dry lubricant is a temperaturesensitive lubricant capable of reacting References Cited by the ExaminerUNITED STATES PATENTS Goetzel 75 20o Denneen et al. 21910.79 Hensel eta1 29182.5 Richardson 75226 REUBEN EPSTEIN, Primary Examiner.

1. A HOT PRESSING APPARATUS WHICH INCLUDES: A SOURCE OF ENERGY; A MOLDCHARACTERIZEDBY AN INTERNAL MOLD CAVITY CAPABLE OF CONTAINING A LOOSELYPACED POWDERED MATERIAL TO COMPACTED; A HEATING COIL IN COMMUNICATIONWITH SOURCE AND ABOUT AND IN A HEAT EXCHANGE RELATIONSHIP WITH THEPOWDERED MATERIAL IN THE MOLD CAVITY; MEANS TO COMPACT THE POWDEREDMATERIAL IN THE MOLD; AND MEANS TO VARY THE LENGTH OF THE HEATING COILIN HEAT EXCHANGE RELATIONSHIP WITH COMPACTION OF THE MATERIAL WHEREBYUNEVEN HEAT DISTRIBUTION DURINGCOMPACTION IS REDUCED.
 7. IN A METHOD OFFORMING A COMPACTED SINTERED ARTICLE WHICH INCLUDES THE STEPS OFCOMPACTING IN A MOLD CAVITY A MASS COMPRISING ELECTRICALLY CONDUCTIVEPARTICLES AND SINTERING SAID PARTICLES DURING COMPACTING BYHIGHFREQUENCY ELECTRICAL ENERGY FROM A COIL PLACED IN AN INDUCTIVERELATIONSHIP WITH THE MASS OF PARTICLES THE IMPROVEMENT WHICH COMPRISES:MAINTAINING A SUBSTANTIALLY CONSTANT INDUCTIVE RELATIONSHIP BETWEEN THECOIL AND THE MASS OF PARTICLES DURING THE COMPACTION AND SINTERING BYVARYING THE FORM OF THE COIL THEREBY PROMOTING MORE EVEN HEATDISTRIBUTION WITHIN THE COMPACTED SINTERED ARTICLE.